At what point does a vented box turn into a MLTL?

[duanebro]

I'd like to know in general when WinISD is to far off to be useful. The case that brought this up is a car audio sub box I am trying to help a man and his son design. They will be using two high power 8" subs. 400 watts each in a 1.6cf box tuned to 34hz. The port is going to be over 40" long and wrap around the box. This brings me back to the original question - is WinISD close enough to use? Any rule of thumb for this time of thing?

Thank you,
Duane

[billfitzmaurice]

Re: At what point does a vented box turn into a MLTL?

- is WinISD close enough to use? Any rule of thumb for this time of thing?

WinISD will give an accurate result but other than the first port resonance it won't predict resonant modes that may arise within the duct. Rule of thumb is to keep the duct shorter and the box larger, so you should model various box sizes to see if a larger box with a shorter duct will give the same or better result without being too large overall. Unless it's intended as a LLT sub you're usually best off with no more than a two foot duct length, give or take.

[edlafontaine]

Re: At what point does a vented box turn into a MLTL?

I think Mass Loaded Transmission Line (MLTL) as a term may be loosely applied to ANY vented box, if one is willing to accept that the term is not necessarily an indicator of the amount of attention to the details which will make ANY vented box successful.
People who use Martin's worksheets are of necessity required to give some consideration to the placement of the elements of the system...I don't think WinISD does that.

It is the placement of those elements which make an enclosure successful.

Martin's worksheets provide feedback in the simulated results for adjustments in the sizing & location of those elements. I don't think WinISD does that to the same extent.

To me, its like driving with a blindfold on. One could start & go, but where would you wind up?

[Paul K.]

Re: At what point does a vented box turn into a MLTL?

Adding somewhat to what Ed said and speaking from lots of experience modeling ML-TLs with Martin's worksheets and building quite a few myself, the 1/4-wave resonant frequency becomes a larger and larger contributor to the overall system tuning frequency as the height or length of the cabinet increases. There won't be very much of this effect when this length is 30" or less (a 30" length has a 1/4-wave resonant frequency of ~110 Hz). But, once you get to 36" and more, it does become enough of a contributor to be considered IMO.
Paul

[duanebro]

Re: At what point does a vented box turn into a MLTL?

Thanks for the fast replies.

Duane

[evilskillit]

Re: At what point does a vented box turn into a MLTL?

Well I think Winisd can calculate first port resonance somewhere if I recall correctly. If the first port resonance is in or near the pass band of the sub then you may hear it. At that point your port is probably too long, but you can try it and see what happens.